This invention relates to polymer thin films, and more particularly relates to polymer thin films having properties like that of bulk polytetrafluoroethylene.
Bulk polytetrafluoroethylene, also known as, e.g., PTFE, (CF.sub.2).sub.n, and Teflon.TM., is characterized by superior mechanical and electrical properties that are important for a wide range of applications. For example, bulk PTFE is characterized by a low dielectric constant of about 2.1 and a low dielectric loss factor of less than about 0.0003 between about 60 Hz and 30,000 MHz. Bulk PTFE is also characterized by high chemical stability, exemplified by its immunity to even strong alkalis and boiling hydrofluoric acid; low water absorption, exemplified by its water uptake of only about 0.005 weight % in a 24 hour period; and high thermal stability, exemplified by its weight loss of only about 0.05 weight % per hour at about 400.degree. C. A low coefficient of friction of between about 0.05-0.08 and a low permeability constant also characterize bulk PTFE. Bulk PTFE is also well-accepted as a substantially bio-compatible material that is tolerable by biological systems such as the human body.
Many biomedical and other applications are not optimally addressed by bulk PTFE, however. For example, biologically-implantable devices such as neural probes, catheter inserts, implantable tubing, and other such devices, all of which are becoming increasingly complicated in geometry, are preferably encapsulated with a film to render the devices impervious to a biological environment, rather than being housed in a bulky PTFE package structure. Such implantable devices typically require of an encapsulating film not only the desired biological compatibility, but due to complex topology and connections to lead wires and associated circuitry, also inherently require an encapsulating film to be conformal and thin, as well as electrically insulating, tough, and flexible. Such a film should further be a good permeation barrier against the implantation environment. A bulk PTFE package structure is thus not optimally applicable to such configurations.
The properties of bulk PTFE are also desirable for thin films employed in the area of microelectronic circuit fabrication. As the desired speed of microfabricated circuits continues to increase, insulating thin films characterized by a correspondingly lower dielectric constant are needed to microfabricate circuit devices having requisite lower characteristic time constants. Additionally, as the functional complexity of microfabricated circuits increases, e.g., with an increasing number of conducting multi-layer interconnects, robust, low-dielectric insulating films are needed to maintain reliable electrical isolation of the multi-layer interconnects as well as to support vias between the layers. Further, robust encapsulation barrier films for microfabrication circuits are becoming increasingly important as circuit applications with harsh operating environments are developed.
There have been proposed various film deposition processes devised with the aim of producing thin films having properties similar to that of bulk PTFE. For example, continuous radio-frequency plasma-enhanced chemical vapor deposition techniques have been proposed for producing PTFE-like films. The films typically produced by such processes have been found, however, to be substantially lacking in one or more critical properties. In particular, the stoichiometry of the resulting films generally differs rather widely from that of bulk PTFE. A typical ratio of fluorine to carbon (F/C ratio) for these films is only about 1.6, whereas bulk PTFE is characterized by a F/C ratio of 2.0. The films produced by various proposed processes are also typically characterized by a low fraction of CF.sub.2 groups; in contrast, bulk PTFE is composed substantially of CF.sub.2 groups. The high degree of crosslinking corresponding to low CF.sub.2 fractionality results in film brittleness, which is unacceptable for applications in which it is desired to encapsulate a flexible, bendable structure in a fluorocarbon film.
Furthermore, carbon-1s X-ray photoemission spectroscopy (XPS) of films deposited by various of the proposed processes reveals that in addition to requisite CF.sub.2 groups, comparable concentrations of unwanted moieties such as CF.sub.3, CF and quaternary carbon moieties are also found in the deposited films. Also, unlike bulk PTFE, films produced by the proposed processes typically contain carbon-carbon double bonds and further contain a significant concentration of dangling bonds. The unpaired electrons of these dangling bonds can be present in concentrations as high as about 10.sup.18 -10.sup.20 spins/cm.sup.3, and result in highly reactive film surface sites.
Taken together, these various unwanted moieties and defects present in typical encapsulating films result in film properties that are sharply degraded below that of bulk PTFE. For example, the dangling bonds in the films, being reactive sites, can react with atmospheric oxygen or water, producing aging effects that result in undesirable film property variations over time. The dielectric properties of the films are also degraded, resulting in, e.g., an increased dielectric loss. The chemical and thermal stability of the films are also degraded, due to the suboptimal film stoichiometry. Thus, deposition techniques devised in an effort to duplicate properties of bulk PTFE for thin-film applications have generally resulted in only suboptimal thin films that typically cannot adequately address performance requirements of PTFE thin film applications.